1. Field of the Invention
The present invention relates to a low-energy intensive process of producing a hexaammine cobalt(III) nitrate complex, Co(NH.sub.3).sub.6 (NO.sub.3).sub.3, from a cobalt(II) salt, and in particular to the production of a hexaammine cobalt nitrate complex which is capable of rapidly oxidizing or decomposing into significant quantities of gases.
2. Description of the Related Art
Hexaammine cobalt nitrate complex Co(NH.sub.3).sub.6 (NO.sub.3).sub.3, (hereinafter referred to as "HACN" for the purposes of brevity) comprises a cationic metal template of cobalt(III), six neutral ammonia ligands and sufficient nitrate oxidizing anion to balance the charge of the complex.
HACN is associated in the art with its ability, when combined with other ingredients, to undergo rapid combustion or decomposition into significant quantities of gases, such as nitrogen gas and water vapor. The combustion or decomposition of HACN-based formulations into significant quantities of gases can be accomplished almost instantaneously by initiating the gas generation reaction with the application of heat or with the use of suitable conventional igniter devices.
The ability of HACN-based formulations to rapidly form gases, and the capability of HACN-based formulations to generate such gases with conventional devices, makes HACN commercially important as, for example, a gas generant component in automobile air bag supplemental restraint systems. In order to comply with automobile manufacturer applications and criteria, HACN gas generant compositions are most often retained in air bag gas generators in the form of pellets or particles. The near instantaneous generation of gases from HACN containing pellets or particles is sufficiently rapid to operatively deploy a collapsed, inflatable air bag within the fraction of a second that elapses from the time when a car is impacted in an accident to the time when the driver or passenger would otherwise be thrust against the steering wheel or dashboard.
The escalation of the commercial significance of gas generant ingredients such as HACN is manifest by the appearance of air bags in many, if not most, new automobiles as the result of government regulations and consumer demand. This increasing commercial demand, however, has placed a greater burden on the development of a synthesis route for preparing HACN and HACN-based compositions in a cost effective and efficient manner.
Various synthesis routes for preparing HACN are known in the art. One synthesis route is described in some detail in Inorganic Syntheses, Volume 2, page 218 (McGraw-Hill, 1946), the complete disclosure of which is incorporated herein by reference. In accordance with this process, HACN is prepared from starting materials such as cobalt(II) nitrate. The cobalt(II) nitrate (0.25 mole of the hexahydrate) is dissolved in water (100 ml) to form an aqueous solution. Then ammonium nitrate (1 mole), activated carbon (0.166 mole) and concentrated ammonia solution (2.5 moles NH.sub.3), are sequentially added to the cobalt solution. The solution is then oxidized by bubbling air through the mixture until the solution turns yellow-brown and a yellow-brown solid forms. This yellow-brown solid is washed with a small amount of ice-cold water and is then dissolved on a steam bath in hot (70.degree. C. to 100.degree. C.) water (1300 to 1500 ml) which is slightly acidified with nitric acid. The activated carbon is then removed by hot filtration. The filtrate is treated with concentrated nitric acid (200 ml) and allowed to cool. The precipitated HACN is again washed with water and alcohol and dried at 100.degree. C. Typical yields of HACN achieved by this process are 88% to 90%.
The activated carbon in the previous literature reaction method is added at a concentration of at least 2.3 wt. % carbon based on the theoretical yield of HACN.
According to F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, pages 773-774 (4th Ed., John Wiley & Sons, 1980), the complete disclosure of which is incorporated herein by reference, a sufficient concentration of the activated carbon is required as a surface-active catalyst to avoid replacement reaction products, such as, for example, [Co(NH.sub.3).sub.5 (NO.sub.3)].sup.2+.
Another literature preparation for hexaammine cobalt(III) salts is described (in "Practical Inorganic Chemistry") (2.sup.nd Ed., G. Pass & H. Sutcliffe, Chapman and Hall, 1982), the complete disclosure of which is incorporated herein by reference. This method employs large amounts of carbon (7.6 wt. %) and hydrogen peroxide as the oxidizing agent. The method further requires that the reaction be heated to an elevated temperature near boiling, for example, 70.degree. C. to 90.degree. C., for a sufficient period to completely oxidize the cobalt(II) salt. Additionally the reaction mixture is then cooled to nearly 0.degree. C. to precipitate the HACN product containing the activated carbon. This carbon-containing product is then isolated and purified by dissolution in a hot (70.degree. C. to 90.degree. C.) nitric acid solution, which is then hot filtered to remove the carbon catalyst. The filtrate is then cooled to nearly 0.degree. C. to precipitate the purified HACN product.
The above-described synthesis routes are plagued by several disadvantages. Since only minor amounts (approximately 1%) of carbon are acceptable as contaminants in the intermediate HACN product for use in a gas generant composition, the hot filtration of the activated carbon is necessary to provide a commercially-acceptable product. The yield of HACN from these synthesis routes is generally no greater than 90% to 95% theoretical. Further, the literature routes are very energy intensive inasmuch as the route requires heating during both the reaction step and the hot filtration step. The energy demands are compounded by the crystallization or isolation steps, one of which is interposed between the reaction step and the hot filtration step. The isolation steps can involve cooling the solutions to freezing temperatures of, for example, about 0.degree. C. In addition to its high energy demands, the practice of this process is also associated with the production of a significant amount of potentially environmentally harmful waste. For example, it has been reported that as much as 20 grams of waste are produced per gram of HACN obtained. Additionally the particle size of the HACN isolated is rather large, typically greater than about 200 microns. For use in gas generant compositions these large HACN particles must undergo a subsequent size reduction step; resulting in a further decrease in yield and additional processing.
A need therefore exists to provide a process for producing HACN which saves energy by avoiding multiple alternating heating and cooling steps, which can be performed with a lesser amount of activated carbon and less expensive reagents, which reduces the amount of the potentially environmentally harmful waste, and in which the resultant HACN product is directly obtainable, in high yield, in particles of acceptable size and purity for use in gas generant compositions.